I really hope Nintendo would use that - alas, they're fiscally conservative, and neither they (nor Bandai-Namco who, IIRC, developed Ultimate's netcode) use rollback. They generally prefer delay-based due to it being cheaper.
That said, rollback does have many major advantages when implemented properly: one of them being that they allow for
much more consistent inputs. That's one of the areas where Ultimate's (official Smash servers in general too) delay-based online system is at its weakest - inputs (and thus attacks) come out much later, and that severely impacts the experience.
While Nintendo could probably implement rollback into Kirby Fighters and I'd argue possibly even Ultimate:
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implementing rollback netcode requires expertise with rollback, it takes a long time as well as loads of resources - even "just" itemless 1v1s - and the resource requirements do increase when factoring in items etc. That's probably the main reason why Sakurai and co. decided not to include rollback in Ultimate. Sakurai mentioned that "
it had a lot of adverse side effects" and I honestly believe that translates to "Those adverse side effects were too much for us to handle in time: if we were to account for all the side effects etc. we'd have to delay the entire game for at least 6-12 months".
I doubt rollback's in Kirby Fighters due to a similar reason, really. It'd take a lot of time to add to a relatively small release. Sure, Them's Fightin' Herds has rollback. but Nintendo might not view a similarily small game as Fighters as "worth it".
RIP Mario.
First choked by an evil space pirate alien.
And now, suplexed by a possibly-secretly-evil black hole of food and cuteness.